1. Field of the Invention
This invention relates to alkali-resistant continuously-drawn glass fibres. Glass fibres of the continuously-drawn type with a high resistance to attack by alkalis are particularly required for use as reinforcement for cementitious products.
2. Description of the Prior Art
There are basically two quite different types of glass fibres: glass wool fibres which are blown from streams of molten glass to attenuate them or which are spun out of apertures in a rotary spinner containing the molten glass, thus forming in either case an entangled wool-like mass of curved discontinuous filaments; and continuously drawn or continuous-filament (C.F.) glass fibres which are drawn as straight continuous filaments from multiple fine apertures in a bushing fed with the molten glass. Numbers of the continuous filaments are combined to form a strand, which may subsequently be chopped to a relatively short length (e.g. 2 inches) but the continuous filament glass fibre retains its distinctive characteristics of being composed of essentially straight parallel filaments. It is this characteristic which renders it suitable for use as reinforcement in composite materials by making impregnation of the fibres relatively easy, whereas glass wool fibres are extremely difficult to use because the random orientation of the filaments make it difficult to produce a uniform composite material without internal voids.
It is known that glass-fibre reinforced cement products can be made, using continuously drawn glass fibres, and that such products, after initial setting, have excellent mechanical strength characteristics, but with the normally available glass fibres made from the conventional composition known as E-glass these strength characteristics deteriorate quite rapidly due to the essentially alkaline nature of cement, particularly ordinary Portland cement, which causes the surface of the fibres to be attacked and their strength accordingly reduced.
To be capable of being successfully and economically drawn into fibres, a glass composition should have a particular combination of physical characteristics which is difficult to achieve in an alkali-resistant composition.
The fibres are drawn continuously from a bushing of platinum or platinum alloy. In order that these bushings should have a sufficient service life to make production of the fibres commercially attractive, it is necessary that the working temperature should not exceed 1350.degree. C, and preferably 1320.degree. C. This working temperature, at which the fibres are drawn from the bushing, is determined by the viscosity of the particular glass composition, which must be approximately 10.sup.3 poises for the production of satisfactory glass fibres. Thus the glass composition should be so selected as to have a viscosity of 10.sup.3 poises at a temperature not exceeding 1350.degree. C and preferably not above 1320.degree. C, this being the working temperature T.sub.W.
As the glass fibres leave the bushing, they cool rapidly due to their high surface area in relation to their volume. In order to avoid crystallisation and consequent weakening or fracture of the fibres, it is necessary that the liquidus temperature of the glass should be at least 40.degree. C below the working temperature, so that the glass reaches a relatively viscous state before it reaches the liquidus temperature and crystallisation is therefore much retarded if not stopped altogether.
The combination of these required characteristics, namely a working temperature (T.sub.W) at which the viscosity is 1000 poises which does not exceed 1350.degree. C, and preferably is not above 1320.degree. C, and a liquidus temperature (T.sub.L) which is at least 40.degree. C less than T.sub.W, is extremely difficult to achieve in a glass which has sufficient alkali resistance to withstand the alkaline environment in cement.
Resistance to alkalis can be imparted by incorporating a proportion of zirconia (ZrO.sub.2) but this tends to result in an increase in the melting temperatures and in the viscosity at such temperatures. A range of glass compositions containing ZrO.sub.2 is disclosed and claimed in U.S. patent application Ser. No. 305,395, now U.S. Pat. No. 3,861,926, in which the viscosity and liquidus temperatures are kept down within acceptable limits by careful choice of the constituents of the glass compositions. It is also known that melting temperatures and viscosity can be reduced by addition of fluorine in the form of a fluoride, but this addition was known to promote crystallisation in bulk glasses. In fact, it is common practice to produce opal glasses with fluorine levels of 2.5% to 4.0% by weight, the opal effect being due to small crystals of alkali metal fluorides and alkaline earth metal fluorides separated out from the vitreous matrix. In the continuous drawing of glass fibres, if such crystallisation took place, it would lead to breakage of the filaments as they were being formed, with serious effects on the production process, so that only small quantities of fluorine were hitherto thought to be permissible. Thus Application Ser. No. 305,395, now U.S. Pat. No. 3,861,926, puts a limit of 2 mol.% on the content of CaF.sub.2.